The present invention relates to a counterflow prevention system to be provided in a fermentation tank, more particularly to a system for preventing a fermentation medium contained in a fermentation tank from being pressed back therefrom owing to a possible pressure increase in the fermentation tank.
It is essentially important, in fermentation industry, that the fermentation tanks in use have their inside, including the fermentation medium or solution contained therein, kept completely free from any kind of sundry microorganism. The work of fermentation may therefore be said to begin substantially with sterilization of the fermentation medium. The sterilization is carried out in general by heating the fermentation medium with a heating medium such as hot water or steam introduced through a sparger installed at the bottom portion of each fermentation tank so as to be immersed in the fermentation medium. The thus sterilized fermentation medium, after being cooled down to an appropriate temperature, is inoculated with purposeful seed microorganisms, and then put into a process of fermentation. Further, in the case of the microorganisms being aerobic, the fermentation medium is aerated with clean air (made free from sundry microorganisms introduced through the above sparger, which has a selective connection with a source of the heating medium and with an air source through piping. Therefore, the fermentation tank has its inner temperature and pressure varied, as is easily supposed, during the above brief sterilization and aeration work, causing a possible pressure increase in the tank to show a tendency of pressing the fermentation medium back into the piping connecting the sparger to the above sources of heating medium and air. The fermentation medium, if it counterflows into the piping, contaminates elements such as operation valves and air filters positioned midway in the piping, and deteriorates the performance of the elements, causing sundry microorganisms to be recurrently grown there.
A conventional method of eliminating such contamination is to provide various instruments for precisely controlling the pressure manually or automatically in the fermentation tank and the piping. According to this method, however, the entire fermentation system is made not only expensive but also complex.
Another conventional method commonly used is to insert a conventional check valve assembly to the piping in series. This method is described in the following with reference to FIGS. 3, 4 and 5.
FIG. 3 shows a conceptual view illustrating the state that a heating medium is supplied to a tank A from a heating medium source (not shown) through a pipe C, a check valve assembly D and a pipe B. The check valve assembly D consists of a first check valve E, an exhaust valve G and a second check valve F, all connected in series with the exhaust valve G positioned between the first and the second check valves E and F. These two check valves E and F are, of course, directed so that the heating medium may flow only in the direction from the heating medium source (not shown) to the tank A. Next, the structure and function of the check valve assembly D are described in detail with reference to FIGS. 4 and 5, which show the states of the assembly allowing the heating medium to flow and of checking the flow of the medium, respectively. According to FIGS. 4 and 5, the check valve assembly D consists essentially of two valve cases 10 and 20 connected to each other at a right angle through a connecting hole 14, two valve plates 11 and 12 vertically movable in the valve case 10, and a valve plate 21 horizontally movable in the valve case 20. The valve plates 11, 12 and 21 are constructed and arranged, by means of their respective springs 11a, 12a and 21a, so as to be forced toward valve sheets 11b, 12b and 12 b formed within the valve cases 11 and 20 in correspondence respectively with the valve plates 11, 12 and 21. Further, the valve case 10 is provided both with an opening to which is connected an inlet pipe C (which corresponds to the pipe C in FIG. 3) so as to face the valve plate 11 and with an exhaust opening 13 so as to face the valve plate 12, while the valve case 20 is provided on its side wall with an outlet pipe B (which corresponds to the pipe B in FIG. 3). In such a structure of the check valve assembly, the valve plate 11 and valve seat form the first check valve E (FIG. 3); the valve plate 12 and valve seat 12b form the exhaust valve G (FIG. 3); and the valve plate 21 and valve seat 21b form the second check valve F (FIG. 3).
With the heating medium supplied through the inlet pipe C, the valve plates 11 and 21 are pressed respectively downward and leftward under the pressure of the medium, and consequentially there is formed in the assembly, as is illustrated in FIG. 5, a passage for the medium, with the exhaust opening 13 being closed which results from the downward displacement of the valve plate 12 having a connection with the valve plate 11. When the supply of the heating medium is stopped, the passage is closed, as is illustrated in FIG. 4, by both the valve plates 11 and 21, with the exhaust opening 13 being open.
According to this check valve assembly, even if the pressure on the outlet pipe side is increased so that the valve plate 21 malfunctions, a possible counterflow from the tank is not only stopped by the valve plate 11 but also exhausted into the atmosphere through the connecting opening 14 and the exhaust opening 13. Further, even when the inlet pipe C has its inner pressure made negative for any reason so that the valve plate 11 also malfunctions, the counterflow from the tank does not flow into the inlet pipe side, though the atmospheric air may be inhaled.
However, also according to a check valve assembly of this type, the assembly can not be prevented from being contaminated by the liquid pressed back from the tank because the liquid comes into contact with the inside walls and members of the assembly.